US 4807283 A Abstract A method for adjusting the balancing impedance of a hybrid junction for maximization of the hybrid attenuation within the range between two operating frequencies, an upper and a lower one, when using a balancing impedance formed by two series-connected resistors (R
_{1}, R_{2}) and a capacitor (C_{1}) connected in parallel with one of said resistors. For the achievement of an optimal matching of the balancing impedance with the impedance of the line wo which the hybrid junction is connected to when all components of the balancing impedance are adjusted, the method according to the invention is effected in such a manner that (a) the first resistor (R_{1}) is adjusted for the maximization of the hybrid attenuation at the upper frequency, (b) the capacitor (C_{1}) is adjusted for the maximization of the hybrid attenuation at the upper frequency, (c) the second resistor (R_{2}) is adjusted for the maximization of the hybrid attenuation at the lower frequency, (d) the first resistor (R_{1}) is readjusted for the maximization of the hybrid attenuation at the lower frequency, (e) the second resistor (R_{2}) is readjusted for the maximization of the hybrid attenuation at the lower frequency, and (f) the capacitor (C_{1}) is readjusted for the maximization of the hybrid attenutation within the whole range of operating frequencies, with an eventual weighing of certain frequencies. (FIG. 8)Claims(5) 1. A method for adjusting balancing impedance in a hybrid junction, said junction comprising a balancing impedance for maximizing hybrid attenuation within a frequency range between predetermined upper and lower operating frequencies; said frequency range being within the passband of a first bandpass filter; said upper frequency determined by the passband of a second bandpass filter; said lower frequency determined by the passband of a third band pass filter; said balancing impedance comprising first and second resistors connected in series and a capacitor connected in parallel with one of said first and second resistors; said method for balancing impedance comprising the steps of:
(a) adjusting said first resistor for the maximization of said hybrid attenuation at said upper frequency; (b) adjusting said capacitor for the maximization of said hybrid attenuation at said upper frequency; (c) adjusting said second resistor for the maximization of said hybrid attenuation at said lower frequency; (d) readjusting said first resistor for the maximization of said hybrid attenuation at said upper frequency; (e) readjusting said second resistor for the maximization of said hybrid attenuation at said lower frequency, and; (f) readjusting said capacitor for the maximization of said hybrid attenuation within said frequency range and weighing frequencies within said range with respect to each other in accordance with the respective frequency responses of said bandpass filters. 2. The method recited in claim 1, comprising the further steps of adjusting said balancing impedance during transmission of a signal.
3. The method recited in claim 1, comprising the further step of determining a magnitude of a received signal and wherein said adjusting and readjusting steps are performed to minimize the determined magnitude of the received signal.
4. The method recited in claim 1, comprising the further steps of:
filtering a signal with said first, second and third bandpass filters, determining magnitudes of the filtered components of the signal, and minimizing the determined magnitudes of the filtered components of the signal by performing said adjusting and readjusting steps. 5. A method for automatically adjusting balancing impedance in a hybrid junction, said junction comprising a balancing impedance for maximizing hybrid attenuation within a frequency range between predetermined upper and lower operating frequencies; said frequency range being within the passband of a first bandpass filter; said upper frequency determined by the passband of a second bandpass filter; said lower frequency determined by the passband of a third band pass filter; said balancing impedance comprising first and second resistors connected in series and a capacitor connected in parallel with one of said first and second resistors; said hybrid junction further comprising control logic for automatically controlling said balancing impedance; said method comprising the following steps, responsive to said control logic:
(a) adjusting said first resistor for the maximization of said hybrid attenuation at said upper frequency; (b) adjusting said capacitor for the maximization of said hybrid attenuation at said upper frequency; (c) adjusting said second resistor for the maximization of said hybrid attenuation at said lower frequency; (d) readjusting said first resistor for the maximization of said hybrid attenuation at said upper frequency; (e) readjusting said second resistor for the maximization of said hybrid attenuation at said lower frequency, and; (f) readjusting said capacitor for the maximization of said hybrid attenuation within said frequency range and weighing frequencies within said range with respect to each other in accordance with the respective frequency responses of said bandpass filters. Description The invention relates to a method for adjusting the balancing impedance of a hybrid junction for the maximization of the hybrid attenuation within a desired range of operating frequencies, preferably within a range between two operating frequencies, an upper and a lower one; the balancing impedance being formed by a first resistor and a second resistor connected in series and a capacitor connected in parallel with one of said first and second resistors. When a four-wire connection is converted into a two-wire connection, a hybrid junction is required for the separation of the transmission directions from each other. In principle, a hybrid junction is a bridge connection which, in an optimal case, is balanced. The balancing impedance thereby equals to the line impedance. The problem lies in the choice of the balancing impedance so that it corresponds to the line impedance in each particular case as the impedance of a transmission line may vary within wide limits, depending e.g. on the cable length, the conductor diameter, the insulating material of the conductor, and the manufacturing tolerances. Consequently, a stable impedance does not even provide a passable result. Optimum balancing has been sought by connecting several different balancing impedances, which has brought some improvement in the situation. This can be done if the used line structures are known and it it is assumed that no manufacturing variation occurs. This, however, is not the case in practice but manufacturing variations may decisively alter the line impedance. Such methods have been tried, too, in which one or more variables of the balancing impedance are adjusted. If a single variable is used, the balancing obtained is merely passable. If several variables are used, the optimum is difficult to find because the variables are dependent on each other. The object of the present invention is to provide a method by means of which the different variables of a multivariable balancing impedance can be adjusted in such a manner that the optimum is always found. This object is achieved by means of the procedure steps defined in the attached claims. The method according to the invention will be described more closely below with reference to the attached drawings, wherein FIG. 1 illustrates a typical impedance of a cable at different frequencies, FIGS. 2, 3, 4, 5, 6 and 7 illustrate the different procedure steps of the method according to the invention, FIG. 8 illustrates one specific circuit structure for the realization of the method according to the invention, FIG. 9 illustrates an alternative structure for the structure of FIG. 8, and FIG. 10 illustrates another specific structure for the realization of the method according to the invention. FIG. 1 illustrates the impedance of a conventional cable at different frequencies within the range from 10 to 100 kHz. It appears from FIG. 1 that the impedance curve can be approximately presented as a portion of a circular arc positioned in that quadrant of the coordinate system which is defined by the +Re and -Im axes thereof. This kind of impedance curve can be approximated by a connection comprising two series-connected resistors and a capacitor which is connected in parallel with one of the resistors. If the cable impedance is indicated by the symbol Z The matching attenuations form acentric circles on the Re-Im plane. It can be seen from the expression that it is the distance from the origin that is important. The same A FIG. 2 illustrates one specific case in which the cable impedance at the upper limiting frequency is indicated by a point Z In the method according to the invention the resistor R In the following step of the method according to the invention, the capacitor C If the impedance point of the cable wire located on the Re-axis, the optimum value of the balancing impedance, too, would be located on the Re-axis. In practice, the impedance point is located in the IV quadrant so that the optimum point is found below the impedance point of the cable (when viewed from the Im-axis). In the following step of the method according to the invention, the resistor R In the following procedure step, the first step is repeated, i.e. the resistor R In the following step of the method, the third procedure step is repeated, i.e. the resistor R The last step of the method comprises adjustment of the capacitor C Even if the method according to the invention has been described above only by means of one specific case, it is to be understood that an optimal matching of the balancing impedance can always be obtained by means of the procedure steps of the method according to the invention. One specific structure for the realization of the method disclosed above will be described below with reference to FIG. 8. In case of FIG. 8 a bridge connection forming the hybrid junction comprises two resistors R, and a balancing impedance which is to be adjusted to the cable impedance Z FIG. 9 illustrates an alternative structure for the structure of FIG. 8, the structure of FIG. 9 corresponding to that of FIG. 8 in all other respects except with regard to the realization of the matching impedance. In order to avoid direct adjustment of the capacitor, i.e. the need of an adjustable capacitor, the adjustement of the capacitor is carried out by doubling the balancing impedance, i.e. the balancing impedance is formed by two impedances arranged in parallel and the capacitors C On the basis of the circuits shown in FIGS. 8 and 9, it is to be understood that the method according to the invention can, in spite of its seeming complexity, be effected by means of a fairly simple and concrete circuits structure realizeable even by means of discrete components. Therefore, the hybrid junction of the invention can be fitted in any modem to automatically effects the maximization of the hybrid attenuation before a connection is made. Simulation results obtained by means of the model of FIG. 8 show, for example, that the method according to the invention enables an attenuation exceeding 40 dB within a typical operating range (80 kbit/s, 160 kbit/s modems) regardless of cable type and manufacturing variations. The simulation also shows that the adjusting accuracy of the parameters is not critical, i.e. the resolution can be kept high. In practice, this implies that the variables A, B and C perhaps require 2 As appears from the afore-described, said n can be e.g. 4 or 5. When the method according to the invention and the circuit structure for the realization thereof are put into practice, the structure of modems presently in use can be greatly simplified. Digital echo supression in particular becomes easier to carry out. Digital echo suppression is simplified because reflections caused by cable mismatching (e.g. different types of cables positioned one after another) are discerned in the hybrid junction as an impedance, which the hybrid junction can at least partially remove. A further advantage of the invention is that it is independent in operation and does not require the use of a processor. Patent Citations
Referenced by
Classifications
Legal Events
Rotate |